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Environmental fate & pathways

Adsorption / desorption

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Description of key information

The adsorption coefficient (Kd) was determined in a test according to OECD TG 106.  The Kd values are in the range  of  940 – 2590 L/kg for three different soils.  The mean value of 2010 L/kg will be used for risk assessment purposes. The substances will adsorb strongly onto the solid phase of soil and sediments due to the cationic surface-active properties. The substance can adsorb both onto the organic fraction and, dependent on the chemical composition, onto the surface of the mineral phase, where sodium and potassium ions can be exchanged against the alkyl ammonium ion. The determination of a Koc from log Kow is not opportune, because the common equations for Koc derivation is not valid for both ionic and surface active substances. 

Key value for chemical safety assessment

Other adsorption coefficients

Type:
other: Kp (solids-water in soil)
Value in L/kg:
2 010
at the temperature of:
22 °C

Other adsorption coefficients

Type:
other: Kp (solids-water in sediment)
Value in L/kg:
2 010
at the temperature of:
22 °C

Other adsorption coefficients

Type:
other: Kp (solids- water in suspended matter)
Value in L/kg:
4 020
at the temperature of:
22 °C

Additional information

Due to the cationic surface-active properties will etherdiamine (N-(3-(Tridecyloxy)propyl)- 1,3-propane diamine, acetate and -(Tridecyloxy)propyl)- 1,3-propane diamine) adsorb strongly onto the solid phase of soil and sediments. The substance can adsorb both onto the organic fraction and, dependent on the chemical composition, onto the surface of the mineral phase, where sodium and potassium ions can be exchanged against the alkyl ammonium ion. The determination of a Koc from log Kow is not opportune, because the common equations for Koc derivation is not valid for both ionic and surface active substances.

The adsorption behaviour of etherdiamine was studied in a batch equilibrium experiment according to a refined OECD 106 (Vos, 2010). Three soils were used Speyer 2.2, loamy sand: 6.4% clay, 2.1% organic carbon; Euro soil 4, silt: 20.3% clay, 1.3% organic carbon and Speyer S6, clay: 42.7% clay, 1.7% organic carbon) encompassing a range of % clay and organic matter. The test substance adsorbed partially onto the container walls which was considered for the determination of the adsorption coefficients. Adsorption kinetics was determined by measurements at different sampling times (up to 24 h), an equilibrium was reached after 3 hours. Desorption occurred to a lesser extent than adsorption. The table below presents a summary of the most important soil properties and observed partitioning constants.

 

Soil properties and related soil partitioning constants:

 

Soil

Clay

Silt

Sand

CEC

pH

Org C

Caq

Kd

Koc

(%)

(%)

(%)

(meq/100g)

(%)

(µg/l)

(102cm3/g)

(104cm3/g)

Speyer2.2

6.4

11.6

82.0

10

5.5

2.09

33.9

9.4

4.5

Eurosoil 4

20.3

75.7

4.1

17.3

6.8

1.31

44.2

25.9

19.8

Speyer6S

42.7

36.1

22.2

22

7.2

1.69

95.7

25

14.8

 

From the data it can be observed that the sorption onto Speyer 6S is much higher than to Speyer 2.2 despite of the higher organic matter content in Speyer 2.2 soil. This can be explained that ionic interactions play a more important role than hydrophobic partitioning with organic matter. Alkyl ammonium ions can interact with the surface of mineral particles or with negative charges of humic substances.

The number of soils which was used in this test deviates from the recommendation in OECD guideline 106 (2000) in that three soils were used instead of the recommended five soils. In addition is the partitioning to soil is not based on a Freundlich isotherm but evaluated based on only one test concentration. These deviations are based on results of earlier adsorption desorption tests with cationic surfactants. The amines in the test substance will to a large extent be protonated under ambient conditions and will therefore interact with the negative surface of mineral particles or with negative charges of humic substances. The ionic interactions play a more important role than hydrophobic partitioning with organic matter. The log Koc is therefore considered as a poor predictor of the partitioning behaviour of cationic surfactants in the environment. These earlier results showed that using three soils with at least one loamy sand and a clay soil, can give as much information as using the full number of soils. These earlier tests also revealed that only rarely linear adsorption isotherms were obtained for cationic surfactants and that extrapolation to lower concentrations based on these non-linear isotherms leads to unrealistic results (e.g. RAR primary fatty amines Oct. 2008). According to the Danish EPA (2004) a more reliable method of extrapolation to lower concentrations, is to use the data originating from the lowest measured concentration and to assume that the coefficient remains constant at lower concentrations. The test as described is therefore performed using only one concentration which is as low as reasonably possible in relation to the detection limit.

The initial concentration used for the determination of the soil partitioning constant were 0.26, 2.5 and 1 mg/L for respectively Speyer 2.2, S6 and Eurosoil 4. The observed aquatic equilibrium concentrations in the experiment range from 34 to 96 µg/L.

For the prediction of the partitioning of the etherdiamine in soil, sediment and suspended matter not the Kd based on organic matter will used but the uncorrected Kd because the relation between the organic matter concentration and the sorption observed alone is not sufficient. Research sponsored by APAG CEFIC is currently performed at University of Leipzich (UFZ, K.U. Goss, S. Droge) and Utrecht (IRAS, J. Hermens) to improve the knowledge on bioavailability and partitioning to soil and sediment.

Because there is no principal difference between soil and sediments considering the sorption properties and because for cationic surfactants the degree of sorption is not related to the organic carbon content, the value for soil will also be used for sediment and suspended particles.

 

In table below the distribution constants (mean values of the 3 tested soils) used in this assessment is summarized:

Table. :Distribution constants for etherdiamine

Kpsoil

2010 L.kg-1

Ksoil-water

3015 m3.m-3

Kpsusp

4020 L.kg-1

Ksusp-water

1005 m3.m-3

Kpsed

2010 L.kg-1

Ksed-water

1005 m3.m-3

 

With a Kpsuspof 4020 L/kg and a concentration of 15 mg/L suspended matter in surface waters, the adsorbed fraction is calculated as ±7%.